The revised International Classification of Epileptic Seizures categorizes absence seizures as generalized seizures, indicating bihemispheric initial involvement both clinically and electroencephalographically.2 Many children with absence seizures (formerly known as "petit mal") can be further categorized as having a characteristic epileptic syndrome.
Types of absence seizures
The terms typical and atypical absence seizures were used by the International Classification of Epileptic Seizures to describe and categorize the various absence types.
Typical absence seizures
The simple typical absence consists of the sudden onset of impaired , usually associated with a blank facial appearance without other motor or behavioral phenomena. This subtype is actually relatively rare. It comprised only 9% of 374 absence seizures video-recorded from 48 patients by Penry and associates.3
Clonic components may be quite subtle. They most frequently consist of eye blinking. Clonic activity may range from nystagmus to rapid jerking of the arms. Changes in tone often include a tonic postural contraction leading to flexion or hypertonic extension. Although a decrease in tone rarely causes a fall, it may lead to nodding of the head or the dropping of objects.
Automatisms are the commonest clinical accompaniment, occurring in 44% of 476 typical absence seizures studied by simultaneous video-EEG radiotelemetry in 27 patients.4 Automatisms are semipurposeful behaviors of which the patient is unaware. The patient cannot recall them afterwards. There are two types:
Autonomic phenomena associated with absence seizures may include:
Neither the autonomic changes nor automatisms allow one to distinguish absence from other seizure types.
Atypical absence seizures
Comparison of typical and atypical absence seizures
Holmes et al5 compared 426 typical and 500 atypical absence seizures in 54 children. The atypical lasted significantly longer, on average, than the typical absence seizure. The most common initial clinical manifestation in either type was a change in facial expression or appearance of a blank stare. A pause or of motor activity was also frequently noted as the initial finding in both types. The atypical absences were significantly more likely to have diminished postural tone or tonic or myoclonic activity as the initial clinical feature.
A blank stare or change in facial expression was the sole clinical finding in only 16% of typical and 28% of atypical absences. Automatisms, eye blinking, and lip smacking occurred more commonly in typical absences. These automatisms were usually perseverative, often consisting of playing with a toy or game. De novo automatisms were associated with longer spells and most commonly consisted of rubbing the face or hands or smiling.
A change in postural tone (either an increase or a decrease) was more commonly seen in atypical absences.
Both typical and atypical absences shared many characteristics:
Although statistically significant differences can be identified, there is considerable overlap between the two seizure types. They most likely represent a clinical continuum. This overlap also pertains to the () and proposed pathophysiology.
Typical absence seizures
The EEG signature of a typical absence seizure is the sudden onset of 3-Hz generalized symmetrical -and-wave or multiple spike-and-slow wave complexes. (See EEG.) The voltage of the discharges is often maximal in the frontocentral regions. The frequency tends to be faster (about 4 Hz) at the onset and slower (down to 2 Hz) toward the end of discharges if they persist longer than 10 seconds. The spike-and-wave discharge may be precipitated by or photic stimulation.
The EEG background is often normal in typical absence seizures. Using the preceding ictal EEG criteria to classify absence seizures, Holmes et al.4 found that only 44% of 27 patients with typical absences had normal EEG backgrounds, however. Diffuse slowing was seen in 22% and paroxysmal spikes or sharp waves in 37%.
The discharges are more numerous during all sleep states except rapid eye movement (REM) sleep. During sleep the bursts are briefer and irregular, slowing to 1.5 to 2.5 Hz.
Hyperventilation, photic stimulation, and hypoglycemia will activate typical absence seizures. Hyperventilation is the most effective procedure.
Clinical effects generally can be seen when discharges last longer than 3 seconds. Detailed neuropsychologic investigations have demonstrated functional impairment from a spike-and-wave burst of any duration.6 Auditory reaction times were delayed 56% of the time when a stimulus was presented at the onset of the EEG paroxysm and were abnormal 80% of the time when the stimulus was delayed 0.5 second. Responsiveness may improve as the paroxysm continues.
Atypical absence seizures
In atypical absence seizures the ictal EEG is more heterogeneous, showing 1.5- to 2.5-Hz slow spike-and-wave or multiple spike-and-wave discharges, which may be irregular or asymmetric (See EEG.).
The interictal EEG background is generally abnormal. In the study of Holmes et al,4 only 11% of 27 patients with atypical absences had a normal interictal EEG. Diffuse slowing and focal or multifocal spikes or sharp waves were seen in 85%.
Adapted from: Holmes GL. Classification of seizures and the epilepsies. In: Schachter SC, Schomer DL, eds. The comprehensive evaluation and treatment of epilepsy. San Diego, CA: Academic Press; 1997. p. 1-36.
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